Currently, scientists believe that dark matter doesn't have much effect on the surrounding environment other than through its gravitational pull. In other words, if two galaxies collide, the dark matter from one galaxy would pass right through the dark matter from the other, as it if didn't exist.

But researchers in Canada and the United States have detected what they are describing as a "dark core" — a large clump of dark matter that appears to have collided and stuck together, even as the galaxies the dark matter was attached to continued to move on.